Ultrafast Vibrational Cooling Inside of a Molecular Container

Vibrational cooling of azulene encapsulated in a hemicarcerand molecular container was studied by pump-probe spectroscopy. Within 1.5 ps of excitation of azulene to the S₁ state, rapid internal conversion through a conical intersection leads to the formation of a vibrationally hot (∼1080 K) ground state, the subsequent cooling of which can be monitored by tracking the evolution of the red-shifted hot band at the edge of the ground-state absorption. It was found that the cooling of the hot S₀ state of azulene in the host-guest complex (hemicarceplex) is 2-4 times faster than that in common organic solvents. Such large acceleration points to a high density of matching vibrational modes and efficient mechanical coupling between the guest and the host. The experimental observations were fully corroborated by the results of molecular dynamics simulations.